Esters of phthalic acid are used to a great extent as plasticizers, in particular for polyvinyl chloride. The principal alcohol components are primary alcohols having 8 to 10 carbon atoms; the most important of these is currently 2-ethylhexanol. Although phthalic esters of shorter-chain alcohols lead to plasticizers having good gelling capacity, their higher volatility is disadvantageous. Longer-chain esters, on the other hand, gel more slowly and have poorer cold resistance.
The properties of the phthalic ester plasticizers are influenced, not only by the size of the alcohol molecule, but also by the branching of the carbon chain. Thus, alcohols having little branching give ester plasticizers having high cold flexibility; substantially linear alcohols having 8 to 10 carbon atoms in the molecule are thus of increased importance as the alcohol components. A precondition for their use is that they are available in large amounts and at favorable cost.
According to German Patent 28 55 421, phthalates of nine carbon alcohols are used as plasticizers and are obtained by oxo reaction of eight carbon olefins, hydrogenation of the reaction product, and esterification of the resultant nine-carbon alcohols with phthalic anhydride. 3% to 20% by weight of the starting olefins have an isobutane skeleton in each molecule chain, less than 3% by weight of the olefins have quaternary carbon, and more than 90% by weight of the total amount of the olefins are present as n-octenes, monomethylheptenes, and dimethylhexenes. In addition, the weight ratio of the total amount of the n-octenes and monomethylheptenes to the dimethylhexenes is more than 0.8.
Phthalic esters based on ten-carbon alcohols are an object of European Patent Application 366,089. These alcohols are used in the form of a mixture which is obtained by hydroformylation of a butene fraction, aldol condensation of the resulting aldehyde mixture, and subsequent hydrogenation. The hydroformylation step is not subject to any restrictions, according to the process description. Thus, cobalt and/or rhodium can be used as catalysts, and the addition of an organic compound of trivalent phosphorus is not excluded.
Another route to obtain didecyl phthalate mixtures is described in European Patent Application 424,767. The preparation of the esters is carried out in a multi-stage process by dimerization of butene mixtures, hydroformylation and hydrogenation of the resulting octene mixture to give a nonanol mixture, dehydration of the nonanol mixture to form a nonene mixture, and hydroformylation and hydrogenation of the nonene mixture, thereby forming the desired decanol mixture.
From economic and industrial aspects, the known processes do not fulfill the requirements of a process carried out on an industrial scale. For example, the starting material may not be available in sufficient quantity, and/or may not be available at favorable costs, and/or the conversion of the starting materials into the alcohols may be associated with processes which are too expensive. In processes which start from butenes which are hydroformylated, the n-valeraldehyde content of the hydroformylation product should, in particular, be as high as possible, in order to promote the formation of straight- or only slightly branched-chain alcohols.